Preserved requested spectral unit
Tidy up initialization of new cubes from modified version of existing cubes.
Set _spectral_unit only in with_spectral_unit
Minor fix
Pass new meta
Fix propagation of spectral unit
Check that WCS auto-changes unit and that spectral_unit is needed
Merge pull request #91 from astrofrog/preserve-spectral-unit

Preserve spectral unit requested
Small improvements to quickstart example
Improvements to docs
Change rest_frequency to rest_value
Don’t show stokes section until it’s written
Added spectral docs section